1. Technical Field
The present invention relates to a method and apparatus for transmitting data for reception by a mobile information device using an orthogonal frequency division multiplexing (OFDM) transmission method, and to an apparatus for receiving signals transmitted by the same method.
2. Background Art
Terrestrial digital broadcasting systems using OFDM modulation are deployed in Europe as DVB-T system and in Japan as ISDB-T system. The European DVB-T system targets primarily fixed receivers, was first deployed as a commercial service in Great Britain in 1998, and is currently used primarily in Europe for commercial service or trial broadcasting services in numerous countries.
This OFDM modulation system is a type of multicarrier modulation. An OFDM signal is generated by dividing and allocating transmission data to multiple mutually orthogonal carriers at the valid symbol period length, using amplitude and phase to add information to each symbol by means of QAM or other multivalued modulation of each carrier, and then applying an inverse Fourier transform to generate the OFDM signal.
Symbol length can be increased in an OFDM signal as a result of using a large number of carriers. A guard interval, which is a period in which a copy of a portion of the signal is cyclically transmitted, can therefore be provided. If a delay wave is in this guard interval, it is removed during FFT processing on the receiver side, and intersymbol interference is thus prevented. This characteristic can be used to construct a SFN (Symbol Frequency Network) in the relay station for relaying signals on the same frequency, and frequencies can thus be used effectively.
Terrestrial transmission channels are significantly inferior to satellite and cable transmission channels, and sequence coding combining convolutional coding and Reed-Solomon coding for error correction is therefore used in the DVB-T system. Byte interleave, bit interleave, and frequency interleave are used for interleaving. FIG. 24 describes DVB-T channel coding.
A hierarchical transmission format is also used in the DVB-T system, and different streams can be assigned to the high bits and low bits of the mapping data. This hierarchical transmission is called mapped hierarchical modulation. The high bits are called high priority (HP) data and the low bits are called low priority (LP) data, and energy dispersion 11, Reed-Solomon coding 12, byte interleaving 13, and convolutional coding 14 are applied to all HP data and LP data. A bit interleave 15 process is then applied bitwise for hierarchical modulation, followed by frequency interleaving 20 and mapping 18. A TPS signal 23 etc. is then inserted and OFDM 17 is applied. Commercial services using hierarchical transmission are not currently offered, however. Only a single channel is used.
DVB-T systems have a 2 k mode and an 8 k mode with 1705 carriers (including 1512 data carriers) and 6817 carriers (including 6048 data carriers), respectively. Frequency interleaving is spread across all valid carriers. The ISDB-T system, however, is not limited to Reed-Solomon coding and convolutional coding, and affords additionally dividing the bandwidth into 13 segments and separately applies frequency interleaving to only the middle segment. ISDB-T can also interleave in the time axis direction.
The ISDB-T system also enables extracting and receiving only the middle segment. Compared with receiving all thirteen segments, this enables lowering the clock speed for the reception process, and as a result enables operating with lower power consumption.    Reference Document: ETSI EN 300 744 V1.4.1 (2001-01) Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television
Because frequency interleaving is applied to the entire frequency band in DVB-T systems, extracting a part of the bandwidth as is possible with ISDB-T is difficult with DVB-T. As a result, instead of segmenting the bandwidth, the necessary data can be transmitted with a mobile terminal only for a specific time in the symbol time direction. In this case it is sufficient to receive only the data transmitted at a specific time, and low power consumption can be achieved. However, data for normal fixed receivers is broadcast in the time band that is not transmitted for mobile terminals. The reception process is thus complicated because data for mobile terminals and data for fixed receivers is intermixed by byte interleaving.
If data for mobile terminals and data for fixed receivers is transmitted without hierarchical modulation in the DVB-T system; both data are necessarily modulated using the same modulation method. However, 64 QAM or other modulation method with a high data transfer rate is used for transmission to fixed receivers due to the large screen size and picture quality demanded. On the contrary, mobile terminals have a small monitor, and MPEG-4 compressed images with a QVGA, CIF, QCIF, or other similarly small size can thus be transmitted to mobile terminals, and a transfer rate of only 250 Kbps to 600 Kbps is required. Fixed receivers can receive signals transmitted with 64 QAM modulation because a roof-top antenna is used for reception. Mobile terminals, however, have a very simple antenna carried at a very low elevation, an 64 QAM reception is thus difficult.
Transmission to mobile terminals and fixed receivers using the same modulation method is therefore not desirable because of the different requirements of mobile terminals and fixed receivers. The present invention relates to a transmission format for sending data for mobile terminal reception without affecting conventional DVB-T receivers.